Circuit for converting unipolar input to bipolar output

Abstract: The circuit for converting unipolar input to bipolar output includes a differential amplifier, first and second feedback resistors; and a peak detector. The negative output of the differential amplifier is fed back to the positive input of the differential amplifier through the first feed-back resistor, and the positive output of the differential amplifier is fed back to the negative input of the differential amplifier through the peak detector and the second feedback resistor. A pole of lowest frequency among feedback amplifier circuits forming the circuit for converting unipolar input to bipolar output is to be determined with cut-off frequency of an amplification stage of the differential amplifier.

Description: The invention relates to a circuit for converting unipolar input to bipolar output, for use in automatic threshold control (hereinafter, it is referred to as "ATC"), for a receiver used in a burst mode digital signal transmission system such as a passive double star optical subscriber transmission system, an optical local area network (generally, referred to as "LAN") or an optical interconnection system. U.S. Patent No. 5,025,456 discloses a circuit for converting unipolar input to bipolar output which uses differential input-output transimpedance amplifier by switching a gain among two gains, as an ATC circuit used in a digital mode optical signal receiver for burst data, for carrying out automatic threshold control starting from a first bit in a burst. Hereinbelow, the operation of a circuit for converting unipolar input to bipolar output is explained in brief with reference to Fig. 1. Fig. 1 illustrates a relationship between the wave shapes of an input and an output of a circuit for converting unipolar input to bipolar output. A circuit for converting unipolar input to bipolar output 50 includes a differential amplifier 1, a peak detector 2 and two feedback resistors 3 and 4. A negative output of the differential amplifier 1 is to be fed back to a positive input of the differential amplifier 1 through the feedback resistor 3, and a positive output of the differential amplifier 1 is to be fed back to a negative input of the differential amplifier 1 through the peak detector 2 and the feedback circuit 4. The differential amplifier 1 connects to a positive input terminal 131, a positive output terminal 132 and a negative output terminal 133. As illustrated in Fig. 2, when the differential amplifier 1 receives a current pulse having an amplitude I in only in the positive direction, namely having a unipolar code, the differential amplifier 1 produces a voltage pulse as a positive output having an amplitude V out + in the positive direction and an amplitude V out - in the negative direction. If the positive amplitude V out + is equal to the negative amplitude V out - , the output voltage has a bipolar code. The circuit for converting unipolar input to bipolar output 50 has two operation modes. When no signals or "L" level signals (signals having lowest input level among input digital signals) are in succession, namely the peak detector is in a reset condition, the circuit 50 enters a condition equivalent to a condition in which the peak detector 2 is short-circuited, at rise-up of a bit, on receiving an "H" level signal (a signal having highest input level among input digital signals). If the feedback resistors 3 and 4 have a common value R F , the differential transimpedance gain is almost equal to R F . This mode is referred to as "Cold Mode" (See Fig. 1). After the bit falls down, the peak detector 2 retains the peak value of the bit. This condition is considered to be equivalent to a condition in which the negative input terminals of the differential amplifier 1 are connected to a constant voltage, so that the differential transimpedance gain is almost equal to 2R F . This mode is referred to as "Warm Mode" (See Fig. 1). By switching these two modes, the circuit for converting unipolar input to bipolar output 50 has specific output voltage wave forms 52 when the input voltage has an amplitude I in . Namely, the circuit 50 has wave forms swinging in the positive direction by I in R F with respect to "H" level signals and wave forms swinging in the negative direction by I in R F with respect to "L" level signals, each I in R F being measured relative to output voltage generated when no signals are generated. Accordingly, by setting decision level "d" a little greater than the level provided when no signals are generated, the circuit for converting unipolar input to bipolar output 50 may have output voltage pulses which never cross the decision level "d" at the approximate center of pulse amplitudes and hence is able to work as an automatic threshold control (ATC) circuit. Fig. 3 shows an example of a conventional circuit for converting unipolar input to bipolar output. Since the circuit illustrated in Fig. 3 is used for a DC-coupled amplifier dealing with very small input signals, it is important to reduce the offset of the bias voltage of the circuit. It is an already known technique to equalize the DC (direct current) characteristics of a differential amplifier with that of a peak detector in order to minimize output offset voltage equivalent to a difference voltage between positive and negative outputs. The aforementioned U.S. Patent No. 5,025,456 explains this technique in detail. In addition, in order to maintain the DC characteristics of a differential amplifier equal to that of a peak detector regardless of the variation of ambient temperatures and supply voltage, a specific circuit had been suggested in the 6th Optical Transmission System Symposium held on December, 1992 in Japan. As illustrated in Fig. 3, this circuit includes a differential amplifier 1 and a peak detector 2. The differential amplifier 1 comprises an emitter-coupled amplification circuit 110 and two emitter follower output stages 130, 140, each stage including a pair of transistors 115, 116 and 117, 118. The transistors 115, 117 are darlington-connected to the transistors 116, 118. The peak detector 2 comprises an emitter-coupled amplification circuit 120 having the same operation point with respect to dc as the emitter-coupled amplification circuit 110, a current blocking transistor 125 a base of which is connected to the positive output of the emitter-coupled amplification circuit 120 and an emitter of which is connected to a hold capacitor 129 for retaining peak values, and a buffer transistor 126 a base of which is connected to an emitter of the current blocking transistor 125 and an emitter of which is connected to the negative input of the emitter-coupled amplification circuit 120. In brief, the differential amplifier 1 had the same amplification stages as those of the peak detector 2 thereby to equalize the DC characteristic of the differential amplifier 1 to that of the peak detector 2. When this circuit is to be used for ATC for receivers in a TDMA (Time Division Multiple Access) system, it is necessary to discharge just sufficiently the hold capacitor provided in the peak detector within the guard time between bursts. As stated in "Journal of Lightwave Technology" Vol. 10, No. 2, page 244, in order to reduce the discharging time thereby to reduce the requisite guard time, a constant current source 150 had been conventionally disposed in parallel with the hold capacitor 129 thereby to enhance the discharging speed. However, the above mentioned conventional circuit for converting unipolar input to bipolar output had problems to be solved as follows. As aforementioned, the differential amplifier 1 has the output stage comprising two pairs of transistors each pair comprising first stage transistors 115, 117 and second stage transistors 116, 118 having darlington connections with the first stage transistors 115, 117 in order to equalize DC characteristics of the differential amplifier to that of the peak detector regardless of variation of ambient temperature and supply voltage. In this circuit, there is a problem that the differential amplifier may work in an identical way with the peak detector due to the parasitic eapacitance of the ground and the emitters of the transistors 115, 117 forming the first stage of the output stage, thereby to reduce the operation speed of the circuit for converting unipolar input to bipolar output. With reference to Figs. 3 and 5, discussed in detail hereinbelow are problems which arise when the aforementioned conventional circuit is used for ATC for receivers in a TDMA system. Fig. 5 illustrates various wave shapes of input and output of a circuit for converting unipolar input to bipolar output, and peak values. Fig. 5-A illustrates input current and Fig. 5-B illustrates output voltages of a peak detector and positive output voltages of an ATC circuit. Among Fig. 5-B, compulsory resets, namely so-called active resets, are not carried out and the leak speed of the peak detector is low in Fig. 5-B-a, while compulsory resets are not carried out and the leak speed of the peak detector is high in Fig. 5-B-b. In Fig. 5-B, the wave shapes of the positive output of the ATC circuit are represented with solid lines, the wave shapes of the output of the peak detector are represented with broken lines, and decision levels are represented with chain lines. As illustrated in Fig. 5-A, when the circuit for converting unipolar input to bipolar output is used as an ATC circuit for receivers in a TDMA system, the circuit for converting unipolar input to bipolar output often receives a burst 2 of small input power level just after receiving a burst 1 of high input power level. In this case, as illustrated in Fig. 5-B-a, if the peak detector retains peak value corresponding to the burst 1 during receiving the burst 2, pulses corresponding to the "H" level of the burst 2 do not reach the decision level. Accordingly, in order to reduce the requisite guard time between bursts, it is necessary to reset the peak detector as soon as the burst has ended and hence return the circuit to the cold mode. As illustrated in Fig. 3, a conventional circuit for converting unipolar input to bipolar output is provided with the constant current source 150 for supplying constant current with the circuit thereby to increase the leak speed of the peak detector 2 and hence reduce reset time. However, as illustrated in Fig. 5-B-b, the use of the constant current source for reducing time to return to the cold mode also makes it easy for the circuit, when "L" level signals are in succession in the burst, to return to the cold mode of the circuit for converting unipolar input to bipolar output when the discharge thereof is completed. The return to the cold mode in the burst causes the decision phase of the cold mode, in which the "H" level input signal intersects the decision level when the "H" level signal rises up, to advance more than the decision phase of the warm mode, so that the clock phase margin of the output of the circuit for converting unipolar input to bipolar output is decreased. In addition, if light is used as input signals, an incorrect extinction ratio of the input signals increases the cases of confusing "L" level with "H" level. Thus, there arises a problem that an optical transmitter to be used has only restricted area of extinction ratio. A conventional circuit for converting unipolar input to bipolar output has different problems from the above mentioned ones. In wave shapes of the circuit for converting unipolar input to bipolar output as illustrated in Fig. 2, the amplitude V out - in the negative direction is usually smaller than the amplitude V out + in the positive direction. In order to nearly equalize the amplitude V out - with the amplitude V out + , it is necessary to make the input resistance of the differential amplifier 1 sufficiently greater than that of the feedback resistors 3,4. In a conventional way, since the circuit illustrated in Fig. 3 is used as a preamplifier of an optical receiver, feedback resistors are arranged as high as possible in order to reduce thermal noises due to the feedback resistors. In order to increase the input resistance of the differential amplifier 1 and reduce noises due to base current in transistors 101, 102, collector resistances 103, 104 need to be arranged to be high. However, if the collector resistances 103, 104 are arranged high, a pole of second lowest frequency approaches a pole of lowest frequency of feedback amplifier circuits, so that the phase margin decreases. Accordingly, a ratio between the input resistance of the differential amplifier 1 and the feedback resistors 3, 4 is in a trade off relationship with the phase margin of the feedback amplifier circuits, and therefore cannot be made greater. Because the ratio cannot be greater, there arises a problem that unipolar and bipolar codes can be converted with less accuracy, namely the amplitude V out - in the negative direction is smaller than the amplitude V out + in the positive direction. In addition, the accuracy of conversion was much dependent on current gain of the transistors 101, 102. It is an aim of the present invention to provide a circuit for converting unipolar input to bipolar output for ATC in order to construct a digital receiver the receiving sensitivity of which has little dependency on supply voltage and ambient temperature due to variation of output offset, and which has a wide bandwidth. It is another aim of the present invention to provide a circuit for converting unipolar input to bipolar output capable of carrying out high speed and stable resets without giving any restriction to successive "L" level signals within a common burst in transmissive signals of a TDMA system. It is still another aim of the present invention to provide a circuit for converting unipolar input to bipolar output for ATC of digital receivers, having high accuracy and low output offset. Features of the invention are defined by the attached claims. An advantage of the invention, is that high frequency current for charging and discharging the parasitic capacitance between the emitter of the transistor and the ground flows into the capacitor disposed in parallel with the transistor, so that a reduction in the operating speed of the circuit, due to a reduction in the operating speed of the emitter follower output stage can be avoided. In addition, with respect to resets of the circuit for converting unipolar input to bipolar output, an embodiment of the invention can switch the leak speed of the peak detector by externally turning on or off the switching circuit, so that the leak speed can be low in a common burst and high between bursts. Furthermore, regardless of ambient temperature and supply voltage, the voltage to which the hold capacitor leaks is coincident with the target voltage generated when the leak has finished, so that stable resets can be performed with a time constant which is a product of a resistance generated when the switch circuit output stage is turned to be on, and a capacity of the hold capacitor. In an embodiment of the invention, a pole of lowest frequency in the feedback and amplifier circuits forming the circuit for converting unipolar input to bipolar output is determined with respect to the cut-off frequency of an amplification stage of the differential amplifier. the output bias level is almost coincident with that of the peak detector, and therefore the output offset can be prevented regardless of ambient temperature and supply voltage. Preferably, a pole of the lowest frequency of feedback amplifier circuits is not dependent on the feedback resistor. Thus, the ratio of input resistance to feedback resistor of the differential amplifier circuit can be made high with a sufficient phase margin, whereby the accuracy in converting the unipolar and bipolar codes can be greatly enhanced. The above and other aims and advantageous features of the present invention will be made apparent from the following description made, purely by way of example, with reference to the accompanying drawings. Fig. 1 illustrates wave shapes showing a principle of a circuit for converting unipolar input to bipolar output; Fig. 2 illustrates wave shapes of input and output of a circuit for converting unipolar input to bipolar output; Fig. 3 is a circuit diagram of a conventional circuit for converting unipolar input to bipolar output; Fig. 4 is a circuit diagram of a first preferred embodiment in accordance with the invention; Fig. 5 illustrates various wave shapes for explaining the operation of the circuit in accordance with the invention; Fig. 6 is a circuit diagram of a second preferred embodiment in accordance with the invention. A preferred embodiment in accordance with the invention will be explained hereinbelow with reference to the drawings. Fig. 4 is a circuit diagram illustrating a first preferred embodiment in accordance with the invention. As illustrated in Fig. 4, this circuit includes a differential amplifier 1 and a peak detector or a peak detecting circuit 2. The differential amplifier 1 comprises an emitter-coupled amplification circuit 110 and two emitter follower output stages 130, 140, each stage including a pair of transistors 115, 116 and 117, 118. The transistors 115, 117 are darlington-connected to the transistors 116, 118. The peak detector or the peak detecting circuit 2 comprises an emitter-coupled amplification circuit 120 having the same operation point with respect to direct current as the emitter-coupled amplification circuit 110, a current blocking transistor 125 a base of which is connected to the positive output of the emitter-coupled amplification circuit 120 and an emitter of which is connected to a hold capacitor 129 for retaining peak values, and a buffer transistor 126 a base of which is connected to an emitter of the current blocking transistor 125 and an emitter of which is connected to the negative input of the emitter-coupled amplification circuit 120. Capacitors 155, 157 are disposed in parallel between a base and an emitter of the first stage transistors 115, 117 each directly connected to the emitter-coupled amplification circuit 110. High frequency current for charging and discharging a parasitic capacitance between an emitter of the transistors 115, 117 and the ground flows into the capacitors 155, 157, resulting in that the reduction of operation speed of the circuit for converting unipolar input to bipolar output due to the reduction of operation speed of the emitter follower output stage can be prevented. In addition, since the peak detector or the peak detecting circuit 2 maintains its DC characteristics unchanged even if the capacitors 155, 157 are added, the DC characteristics of the differential amplifier are kept to be identical with those of the peak detector regardless of the variation of ambient temperature and supply voltage. Accordingly, it is possible to obtain a circuit for converting unipolar input to bipolar output for ATC having small dependency of receiving sensitivity on supply voltage and ambient temperature, and making it possible to form a digital receiver having wide bandwidth. Hereinbelow, with reference to Figs. 1 and 2, the operation of the reference voltage producing circuit 5 and the switching circuit 6 is described. Among Fig. 5-B, compulsory resets are carried out and the reference voltage is greater than the target voltage in Fig. 5-B-c, while compulsory resets are carried out and the reference voltage is smaller than the target voltage in Fig. 5-B-d, and compulsory resets are carried out and the reference voltage is equal to the target voltage in Fig. 5-B-e. In Fig. 5-B, the wave shapes of the positive output of the circuit for converting unipolar input to bipolar output are represented with solid lines, the wave shapes of the output of the peak detector are represented with broken lines, and decision levels are represented with chain lines. In the reference voltage producing circuit 5, an average of the positive output and the negative output of the emitter-coupled amplification circuit 110 is detected by a voltage division circuit 111. The level of this average is shifted by the transistors 113, 114 and then the average is introduced to an impedance convertor or an impedance converting circuit 7 from the emitter of the transistor 113. The output stage of the switching circuit 6 comprises JFET 8, which is turned on or conductive when the voltage applied to an input terminal 134 is "L", while turned off or nonconductive when the voltage applied to the input terminal 134 is "H". Since the time when a burst ends can be known in a TDMA transmission system, reset signal pulses with a duration of τ can be obtained externally of the circuit for converting unipolar input to bipolar output within a guard time, and be applied to the terminal 134 to which the reset signals are to be input. It should be noted that pulse durations τ of the reset signals should be greater than a time constant which is equivalent to a product of the resistance generated when JFET 8 is turned on and the capacity of the capacitor 129. In this embodiment, JFET 8 is turned on or conductive only within a time interval τ in which the voltage applied to the input terminal 134 is "L", while JFET 8 is turned off or nonconductive within time other than the time intervals τ. In the time other than the time intervals t, voltage applied to the input terminal 134 is "H". As illustrated in Fig. 5-B-e, since the leak speed of the peak detector is arranged to be slow enough, the peak detector 2 maintains a peak value of pulses forming the burst 1 even if "L" level signals are in succession within a series of pulses in the burst 1. Thus, the possibility is low at which the circuit for converting unipolar input to bipolar output returns to the cold mode in the burst 1. Accordingly, the problems that the clock phase margin of the output of the identification circuit is reduced and that the extinction ratio of optical transmitters is restricted can be solved. By applying reset signals having time intervals τ to the input terminal 134 as soon as the burst 1 ends, JFET 8 is turned to be conductive only within a period τ and the capacitor 129 retaining electric charges corresponding to a peak value in the peak detector 2 is discharged to the reference voltage producing circuit 5. Since the pulse duration τ of the reset signals is greater than the time constant defined with a product of the resistance generated JFET 8 is turned to be on and the capacity of the capacitor 129, the discharge ends within the time interval τ and the peak detector 2 is reset thereby to cause the circuit for converting unipolar input to bipolar output to return to the cold mode. Though the output voltage of the peak detector 2 when the discharge has ended is equal to the output voltage of the reference voltage producing circuit, the latter output voltage has to be equal to a voltage (discharge target voltage) when the discharge ends by natural discharge due to continuous input of no signals. When the output voltage of the reference voltage producing circuit 5 is higher than the target voltage, as illustrated in Fig. 5-B-c, sufficient reset is not carried out. Accordingly, pulses corresponding to an "H" level of the burst 2 do not reach the decision level. On the other hand, when the output voltage of the reference voltage producing circuit 5 is higher than the target voltage, as illustrated in Fig. 5-B-d, excessive discharge occurs and hence pulses corresponding to an "L" level of the burst 2 may exceed the decision level. However, in this circuit in accordance with the embodiment, since the differential amplifier 1 has the same DC characteristics as the peak detector 2, the output voltage of the voltage division circuit 111 is equal, regardless of the presence or absence of input signals, to the output voltage of the emitter-coupled amplification circuit 120 generated when no signals are input. In addition, since the transistors 113, 114 have the same DC characteristics as those of the transistors 125, 126 respectively and the transistor 114 has the same drive current as the transistor 126, the voltage between a base and an emitter of the transistor 113 is equal to the voltage between a base and an emitter of the transistor 125. Accordingly, the output voltage of the emitter of the transistor 113 can be considered to be equal to the output voltage of the emitter of the transistor 125 generated when no signals are input, namely the discharge target voltage. By operating the impedance convertor 7 using the output voltage of the emitter of the transistor 113 as input signals, the output voltage of the reference voltage producing circuit 5 can be maintained to be equal to the target voltage regardless of the presence or absence of the input signals and the variation of ambient temperature and supply voltage. Thus, stable resets are carried out at high speed without problems occurring when the circuit for converting unipolar input to bipolar output returns to the cold mode while "L" level are in succession in a common outburst in transmission signals of a TDMA system, and also without dependency on the variation of ambient temperature and supply voltage. In this embodiment, a JFET is used in the output stage of the switching circuit; however, other elements may be used instead such as a switching element including a bipolar transistor and MOSFET or a combination thereof. Fig. 6 is a circuit diagram of a second embodiment of the invention. The negative output of the differential amplifier 1 is fed back to the positive input thereof through the feedback resistor 4, and the positive output of the differential amplifier 1 is fed back to the negative input thereof the peak detector 2 and the feedback resistor 3. The differential amplifier 1 comprises an input buffer stage 307, an amplification stage 308 and a level shift circuit. The input buffer stage 307 comprises an emitter-coupled amplification circuit having an emitter degeneration resistor therein, and has sufficiently high input resistance by disposing the emitter degeneration resistor. The amplification stage 308 comprises an emitter-coupled amplification circuit, and has capacitors 417, 418 between bases and collectors of transistors 415, 416 thereby to narrow the bandwidth of the transistors 415, 416. The narrow banding by means of the amplification stage 308 and the reduction of the feedback resistors 3, 4 makes it possible that a pole of the lowest frequency among feedback amplifier circuits forming a circuit for converting unipolar input to bipolar output is determined with a cut off frequency of the amplification stage of the differential amplifier, and that a pole of the second lowest frequency is determined with a cut off frequency of an input section of the circuit for converting unipolar input to bipolar output. Accordingly, the phase margin does not decrease if the feedback is increased by reducing the feedback resistor. Since the input buffer stage 307 makes it possible to enhance the input resistance of the differential amplifier 1 and the limit of the bandwidth in the amplification stage 308 makes it possible to reduce the feedback resistors 3, 4, the input resistance of the differential amplifier 1 can be made sufficiently higher than the feedback resistors 3, 4 thereby to enhance the accuracy in converting unipolar and bipolar codes relative to the conventional circuits. The accuracy in converting unipolar and bipolar codes is dependent on current gain β of transistors disposed in the input stage of the differential amplifier 1. In this sixth embodiment, the input resistance is about 50 kilo ohms when the feedback resistor is one kilo ohm and the current gain β is 100. Even if the current gain β is reduced by half, the input resistance is about 25 kilo ohms and hence is sufficiently higher than the feedback resistor. Thus, the accuracy in converting the unipolar and bipolar codes is little dependent on the current gain β. In this second embodiment, the emitter-coupled amplification circuit disposing the emitter degeneration resistor therein is used as an input buffer stage; however, other circuits may be used instead, such as an emitter follower circuit capable of obtaining high input resistance. The limitation of bandwidth is carried out in the sixth embodiment by disposing a capacitor(s) in an amplification stage between a base(s) and a collector(s) of a transistor(s) constituting an emitter-coupled amplification circuit, but may be carried out in other ways. As aforementioned, the invention provides a circuit for converting unipolar input to bipolar output for ATC having little dependency of receiving sensitivity on supply voltage and ambient temperature and contributing to fabricating a digital receiver having wide bandwidth and high accuracy and consuming only low power. In addition, the invention can carry out speedy and stable resets without giving limitation to the succession of "L" level in a burst in transmission signals in a TDMA system . Thus, the invention provides a quite useful circuit for converting unipolar input to bipolar output. It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims: A circuit for converting unipolar input to bipolar output comprising: a differential amplifier (1) including a first emitter-coupled amplifying circuit (110) and an emitter follower output stage (130,140), said emitter follower output stage (130,140) including two pairs of transistors (115,116; 117,118) each pair comprising a first stage transistor (115,117) and a second stage transistor (116,118) having a darlington connection with said first stage transistor; a peak detector (2) including a second emitter-coupled amplifying circuit (120) having an identical DC operation point with said first emitter-coupled amplifying circuit (110), a current blocking transistor (125) a base of which is connected to an output of said second emitter-coupled amplifying circuit (120) and an emitter of which is connected to a hold capacitor (129) for maintaining peak values, and a buffer transistor (126), a base of which is connected to an emitter of said current blocking transistor (125) and an emitter of which is connected to an input of said second emitter-coupled amplifying circuit (120); and first and second feedback resistors (3,4); one of the negative output or positive output of said differential amplifier (1) being fed back to one of the positive input or negative input of said differential amplifier through said first feedback resistor (3), and the other of the negative output or positive output of said differential amplifier (1) being fed back to the other of the positive input or negative input of said differential amplifier through said peak detector and said second feedback resistor (4), and a capacitor (157,155) disposed in parallel between the base and emitter of at least said first stage transistor (115,117) connected to said first emitter-coupled amplifying circuit (110) among said pair of transistors (117,118) A circuit for converting unipolar input to bipolar output in accordance with claim 1 further comprising: a reference voltage producing circuit (5) for producing a reference voltage equivalent to an average of the positive output and the negative output of said emitter output stage of said first stage transistor (115,117); and a switching circuit (6) capable of being externally turned on or off, disposed between an emitter of said current blocking transistor (125) and said reference voltage producing circuit (5). A circuit for converting unipolar input to bipolar output in accordance with claim 2 wherein said reference voltage producing circuit (5) comprises a voltage divider (111) for obtaining an average of the positive output and negative output of said first emitter-coupled amplifying circuit (110) by means of voltage division; a darlington pair (113,114) including two stages of transistor, a base of a first stage transistor (113) being connected to said voltage divider (111); and an impedance convertor (7) connected to an emitter output of said first stage transistor (113) of said darlington pair (113,114). A circuit for converting unipolar input to bipolar output according to claim 2 or 3 wherein an output stage of the switching circuit (6) includes a JFET (8) which is externally turned on or off.

Inventors: ANZAI NORIKO, NAGAHORI TAKESHI, OAMI TOSHIMASA, ANZAI, NORIKO, NAGAHORI, TAKESHI, OAMI, TOSHIMASA, ANZAI, NORIKO, C/O NEC TELECOM SYSTEMS, LTD., NAGAHORI, TAKESHI, C/O NEC CORPORATION, OAMI, TOSHIMASA, C/O NEC TELECOM SYSTEMS, LTD.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H04L25
IPC codes: H04L25, H04B10